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Carbon monoxide poisoning

Page last reviewed: 13/07/2011

Carbon monoxide (CO) gas is produced by the incomplete burning of fuel. It is poisonous, and even breathing in a small amount can cause loss of consciousness and death. In Ireland about 40 people die from accidental carbon monoxide poisoning every year.

How is carbon monoxide produced?

Carbon monoxide is hard to detect because it has no smell, taste or colour. This also means that it is easy to inhale without realising.

Carbon monoxide is produced when fuels such as gas, oil, coal and wood do not burn fully. When a fire burns in an enclosed room, the oxygen in the room is gradually used up and replaced with carbon dioxide. Following a build-up of carbon dioxide in the air, the fuel is prevented from burning fully and it starts to release carbon monoxide.

The effects of breathing in carbon monoxide

When you breathe in carbon monoxide it enters your bloodstream. There it mixes with haemoglobin. Haemoglobin is the part of red blood cells that carries oxygen around your body. However, when carbon monoxide mixes with haemoglobin, the blood is no longer able to carry oxygen. The lack of oxygen causes the body's tissue and cells to die.

When haemoglobin mixes with carbon monoxide it produces a compound called carboxyhaemoglobin. Carboxyhaemoglobin adversely affects blood vessels in the body, causing them to become leaky. This can lead to swelling in the brain, causing unconsciousness and neurological (nerve) damage.


People with mild symptoms of carbon monoxide poisoning usually make a full recovery.

However, between 10 and 50% of people with serious CO poisoning can have long-term problems. Sometimes, complications can arise years later as a result of the CO gas causing damage to the heart.

Oxygen is an odourless, colourless gas that makes up about 20% of the air we breathe.
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Body tissue is made up of groups of cells that perform a specific job, such as protecting the body against infection, producing movement or storing fat.
Blood vessels
Blood vessels are the tubes in which blood travels to and from parts of the body. The three main types of blood vessels are veins, arteries and capillaries.
The brain controls thought, memory and emotion. It sends messages to the body controlling movement, speech and senses.

Page last reviewed: 13/07/2011

A headache is the most common symptom of carbon monoxide (CO) poisoning.  Other common symptoms include:

  • feeling sick (nausea) and dizziness
  • feeling tired and confused
  • being sick (vomiting) and having abdominal (stomach) pain
  • shortness of breath and difficultly breathing (dyspnoea)

The symptoms of CO poisoning can resemble those of food poisoning and the flu. However, unlike flu, CO poisoning does not cause a high temperature (fever).

You may notice that your symptoms are less severe at times when you are away from the source of the carbon monoxide.

The longer you breathe in CO gas, the worse your symptoms will get. You may lose your balance, vision and memory. Eventually, you may lose consciousness.

This can happen within two hours if there is a lot of CO in the air. However, the symptoms of CO poisoning can sometimes occur a number of days or months after breathing in carbon monoxide.

Symptoms of CO poisoning that develop later include:

  • confusion
  • memory loss
  • co-ordination problems

High levels of carbon monoxide

If you have breathed in high levels of carbon monoxide, the symptoms can be much more severe and can include:

  • intoxication (the effects of poisoning, such as an impaired mental state) and changes to personality
  • vertigo (the feeling that you, or the environment around you, are spinning)
  • ataxia (loss of physical co-ordination due to underlying damage to the nervous system and brain)
  • breathlessness and tachycardia (a heart rate of more than 100 beats a minute)
  • chest pain due to angina or heart attack
  • seizures (an uncontrollable burst of electrical activity in the brain that causes muscle twitches, tongue biting and total body shaking)
  • loss of consciousness (in cases where there are very high levels of carbon monoxide, death may occur within a few minutes)

At risk groups

Certain people in your household may be affected by CO poisoning more quickly than others. Those at particular risk include:

  • people with heart or breathing problems
  • babies and young children
  • pregnant women

Pets may be the first to show signs of CO poisoning because they are very vulnerable to the effects of CO gas. The smaller an animal or a person is, the faster CO will affect them.

If your pet suddenly becomes ill or dies unexpectedly, and the death is not related to old age or an existing condition, you should investigate the possibility of a CO leak.


Pain is an unpleasant physical or emotional feeling that your body produces as a warning sign that it has been damaged.
High temperature
A high temperature, also known as a fever, is when someone's body temperature goes above the normal 37°C (98.6°F).
Vomiting is when you bring up the contents of your stomach through your mouth.
Nausea is when you feel like you are going to be sick.
The heart is a muscular organ that pumps blood around the body.

Page last reviewed: 13/07/2011

Gas, oil, coal and wood are all fuel sources that are used in many household appliances, including:

  • boilers
  • gas fires
  • central heating systems
  • water heaters
  • cookers
  • open fires

If the fuel in these appliances does not burn fully, carbon monoxide (CO) gas is produced. Burning charcoal, running cars and smoking cigarettes also results in the production of CO gas.
The main causes of CO poisoning are outlined below.

Poorly installed or maintained appliances

Household appliances, such as cooking and heating devices, that are incorrectly installed and badly maintained are the main causes of accidental exposure to carbon monoxide.

Provided that household appliances are correctly fitted, used safely and well maintained, they should produce very little CO gas. Damaged appliances, or those that are not serviced regularly, often produce higher levels of CO gas than normal and become dangerous.

Blocked flues and chimneys

Blocked flues and chimneys are another potential cause of carbon monoxide poisoning as they can stop CO gas escaping, allowing it to build up to dangerous levels in a room.

Enclosed or unventilated spaces

Burning fuel in an enclosed or unventilated space, where there are no air vents, windows or doors left open or ajar, increases the risk of CO poisoning, for example, a car engine that is left running inside a garage, or a faulty heating boiler in a poorly ventilated kitchen. If a car engine is left running in a closed garage, a lethal level of CO can build up in around 10 minutes.

Paint fumes

Fumes from cleaning fluids and paint removers that contain methylene chloride (dichloromethane) can also cause CO poisoning. When it is inhaled (breathed in) methylene chloride is converted into carbon monoxide.

Page last reviewed: 13/07/2011

Your GP will diagnose carbon monoxide (CO) poisoning based on a combination of physical and environmental factors such as:

  • your symptoms
  • information relating to your household, such as your heating and cooking appliances

If you have the symptoms of CO poisoning, a blood sample will be taken to check the amount of carboxyhaemoglobin in your blood. A level of 30% indicates severe exposure.

An electrocardiogram (ECG) may also be carried out to investigate how well your heart is pumping blood around your body.

Be aware of the signs and symptoms

It is very important to be aware of the dangers of CO poisoning, and to look out for the warning signs.

As well as the effects of CO gas that are described in Carbon monoxide poisoning - symptoms, the following situations can also be signs of CO poisoning:

  • other people in your house, flat or workplace who have similar symptoms
  • your symptoms disappear when you go away on holiday and return when you come back
  • your symptoms tend to be seasonal, for example, you get headaches more often during the winter when the central heating is used more frequently

Other possible signs of a CO leak in your home include:

  • black sooty marks on the front cover (radiants) of gas fires
  • sooty marks on the wall around boilers, stoves or fires
  • smoke building up in rooms as a result of faulty flues
  • yellow instead of blue flames coming from gas appliances

Stop using all gas appliances

If several people in the same building develop flu-like symptoms without a temperature, and you think it could be linked to a CO leak, you should:

  • immediately stop using all your cooking and heating appliances that use fuel other than electricity
  • open all of the windows in your house or building
  • move away from the source of the CO gas
  • contact your local gas supplier
  • visit your GP as soon as possible

If you have a CO leak, ask a suitably qualified engineer to inspect your cooking appliances, central heating and water heating appliances, to check that they are safe.


An ECG (electrocardiogram) is a test that measures electrical activity in the heart, and is used to identify heart problems. 
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
The heart is a muscular organ that pumps blood around the body.

Page last reviewed: 13/07/2011

If it is suspected that you have carbon monoxide (CO) poisoning, you should move away from the possible source of the gas so that your symptoms can be assessed.

Consult your GP immediately if you think that you have CO poisoning. If you have mild CO poisoning, you will probably not have to go to hospital, but it is important that you still seek medical advice.

Standard oxygen therapy

If you have been exposed to a high amount of CO gas, you will be treated in hospital. You will be given 100% oxygen through a tight fitting mask (normal air contains around 21% oxygen).

Breathing in concentrated oxygen enables your body to replace carboxyhaemoglobin more quickly. You will continue to have oxygen therapy until your carboxyhaemoglobin levels decrease to a level that is below 10%.

Hyperbaric oxygen therapy

Hyperbaric oxygen therapy (HBOT) is a treatment that saturates (floods) the body with pure oxygen, helping it to overcome the oxygen shortage that is caused by CO poisoning.

There is currently insufficient evidence regarding the long-term effectiveness of HBOT for treating severe cases of CO poisoning. Therefore, standard oxygen therapy (as described above) is usually the recommended treatment option.

However, in certain situations, HBOT may be recommended. For example, it may be used in cases of extensive CO exposure and if nerve damage is suspected. The decision to carry out this treatment will be decided on a case by case basis.


An ECG (electrocardiogram) is a test that measures electrical activity in the heart, and is used to identify heart problems. 
The brain controls thought, memory and emotion. It sends messages to the body controlling movement, speech and senses.
A coma is a sleep-like state when someone is unconscious for a long period of time.
Oxygen is an odourless, colourless gas that makes up about 20% of the air we breathe.

Page last reviewed: 13/07/2011

Carbon monoxide (CO) gas is poisonous, and a high amount of CO in the air presents a danger to everyone.

However, certain groups of people, such as babies and young children, pregnant women, those with chronic heart disease, and those with respiratory problems, such as asthma, are even more vulnerable to the effects of CO gas.

CO poisoning can be mild, leading to flu-like symptoms, such as headache and dizziness, or it can be more serious. The severity of CO poisoning depends on the length of exposure and how much carbon monoxide gas you have been exposed to.

Between 10 and 15% of people who have severe carbon monoxide poisoning develop long-term complications. Some of the complications of severe exposure to carbon monoxide gas are described below.

Permanent brain damage

Prolonged exposure to CO gas can cause memory problems and difficulty concentrating. It can also cause vision and hearing loss.

In rare cases, people who have been exposed to CO gas over a long period of time can develop Parkinsonism. This is a condition that is characterised by the main symptoms of Parkinson's disease: shaking, stiffness and slowness of movement.

Parkinson's disease is a chronic (long-term) neurological (brain) condition that affects the way that the brain co-ordinates body movements, including walking, talking and writing.

See the Health A-Z topic about Parkinson's disease for more information.

Heart damage

Coronary heart disease can occur as a result of many years of CO poisoning.

See the Health A-Z topic about Coronary heart disease for more information about the condition.

Urinary incontinence

Urinary incontinence is a condition where you unintentionally pass urine. It is sometimes described as an involuntary leakage of urine. Urinary incontinence is particularly common in women.

Harm to unborn babies

Exposure to CO gas may cause harm to an unborn child. Low birth weight, perinatal death (stillborn and deaths within the first four weeks of birth), and behavioural problems may occur in children who are exposed to CO during pregnancy.

Page last reviewed: 13/07/2011

The best way to protect both you and your family from carbon monoxide (CO) poisoning is to be aware of the dangers and to identify the appliances that could emit CO gas.

Be aware of the early warning signs of CO poisoning, and look out for the signs and symptoms.

Follow the guidelines listed below to protect yourself in your home and workplace.

  • Make sure that your household appliances are safe and well maintained.
  • Boilers, cookers, heating systems and appliances should be installed and regularly serviced by a reputable, registered engineer.
  • Anyone carrying out work on gas installations and appliances in your home must be on a Register.
  • Never use ovens or gas ranges to heat your home.
  • Make sure that rooms are well-ventilated, and do not block air vents. If your home is double-glazed or draught-proofed, make sure there is still enough air circulating for any heaters that are in the room.
  • Make sure that all chimneys and flues are swept from top to bottom regularly and that they are kept clear (at least once a year) by a qualified sweep.
  • Do not use gas-powered equipment and tools inside your home if you can avoid it. Only use them in a well-ventilated area and put the engine unit and exhaust outside.
  • Always use a safety mask when using chemicals that contain methylene chloride.
  • Do not leave petrol-fuelled lawnmowers or cars running in the garage.
  • Do not burn charcoal in an enclosed space, such as on an indoor barbecue.
  • Do not sleep in a room that has an unflued gas fire or a paraffin heater.
  • Fit an extractor fan in your kitchen (if it does not already have one).

CO alarms

The most reliable way of checking CO levels in your house is to install an audible CO alarm. CO alarms are available from DIY and hardware stores. There are several different types of CO alarm.

CO alarms should give out a high-pitched noise when levels of CO are high. However, you should never rely on them entirely because they are a warning system and not a replacement for regularly servicing household appliances.

When buying a CO alarm, make sure that it is approved to the latest British or European Standard (BS Kitemark or EN50291).

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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